1. Field of the Invention
The present invention relates to a nonreciprocal circuit device and a communication apparatus using the same.
2. Description of the Related Art
In general, a nonreciprocal circuit device such as a lumped-constant isolator serves to pass signals only in a transmission direction and to block signals in the opposite direction. Such a type of isolator will be described with reference to FIGS. 13 and 14. As shown, an isolator 200 includes a permanent magnet 209, a ferrite element 210 to which a DC magnetic flux is applied by the permanent magnet 209, a plurality of center electrodes 220 provided on the ferrite element 210, and matching capacitance elements C connected to respective ends of the center electrodes 220. The isolator 200 also has an upper casing member 208 and a lower casing member 204 which are made of magnetic metal and which accommodate the permanent magnet 209, the ferrite element 210, the center electrodes 220, and the matching capacitance elements C. The upper casing member 208 and the lower casing member 204 are configured to have the same thickness t (typically 0.2 mm).
As shown in FIGS. 15 and 16, in the isolator 200, the permanent magnet 209, the ferrite element 210, the upper casing member 208, and the lower casing member 204 constitute a magnetic circuit. The DC magnetic flux is uniformly applied by the permanent magnet 209 to the ferrite element 210.
While such an isolator 200 has been successfully incorporated in a mobile communication apparatus such as a portable telephone, there is a need for further reduction in size.
Accordingly, the present invention provides a nonreciprocal circuit device that is reduced in size and particularly in height.
To this end, according to a first aspect of the present invention, there is provided a nonreciprocal circuit device. The nonreciprocal circuit device includes a permanent magnet, a ferrite element to which a DC magnetic flux is applied by the permanent magnet, and a plurality of center electrodes provided on the ferrite element. The nonreciprocal circuit device further includes a metal casing which is made of an iron-based metal and which accommodates the permanent magnet, the ferrite element, and the center electrodes. The metal casing includes a first casing member and a second casing member. The first casing member and the permanent magnet are in magnetic contact with each other. The thickness of the second casing member is in the range between 50% and 100% of the thickness of the first casing member.
With this arrangement, almost the entire DC magnetic flux of the permanent magnet flows through the first casing member that is in magnetic contact with the permanent magnet. The term xe2x80x9cmagnetic contactxe2x80x9d herein refers to the case in which the permanent magnet is in direct contact with the first casing member as well as the case in which the permanent magnet is attached to the first casing member by means of an adhesive (non-magnetic material) or the like. On the other hand, only part of the DC magnetic flux of the permanent magnet flows through the second casing member that is not in magnetic contact with the permanent magnet, due to the occurrence of a leakage flux. Thus, the thickness of the second casing member can be reduced to be within the range between 50% and 100% of the thickness of the first casing member, that is, within the range in which the DC magnetic flux flowing through the second casing member is not saturated. This arrangement, therefore, can provide a nonreciprocal circuit device that is reduced in size and particularly in height.
In one form of the invention, the second casing member has a pair of second casing sidewalls that oppose each other. In this case, the end surfaces of the first casing member are butted and joined to the sidewalls of the second casing member. Since the first casing member has no sidewalls, this arrangement can reduce the width of the nonreciprocal circuit device.
In another form of the invention, the first casing member has a pair of first casing sidewalls that oppose each other, and the second casing member has a pair of second casing sidewalls that oppose each other. In this case, the first casing sidewalls and the second casing sidewalls are overlapped and joined to each other. This arrangement can facilitate the assembly of the nonreciprocal circuit device and can stabilize the positional relationship of the first and second casing members after assembly. This arrangement, therefore, can provide a nonreciprocal circuit device with improved frequency characteristics.
Preferably, the nonreciprocal circuit device further includes a resin casing member which is incorporated in the metal casing and which accommodates the ferrite element and the center electrodes. The resin casing member has contact-preventing portions that extend therefrom. Each of the contact-preventing portions is provided between the inner surfaces of the first and second casing sidewalls and the peripheral surface of the permanent magnet. Thus, the contact between the inner surfaces of the sidewalls of the metal casing and the peripheral surface of the permanent magnet can be prevented. This arrangement, therefore, stabilizes the magnetic field distribution formed by the permanent magnet, that is, the DC magnetic flux applied to the ferrite element, which can provide a nonreciprocal circuit device with stable electric characteristics.
Preferably, the second casing member and the resin casing member are integrally formed. This arrangement can provide an enhanced accuracy in positioning the resin casing member relative to the second casing member, allowing improvements in the assembly characteristics and the assembly efficiency of the nonreciprocal circuit device.
Preferably, the first casing member and the second casing member are joined by welding. This improves the efficiency of the magnetic circuit formed in the metal casing, allowing a reduction in size of the metal casing. This arrangement, therefore, can provide a nonreciprocal circuit device that is reduced in size.
Preferably, the surfaces of at least one of the first casing member and the second casing member are plated with one of nickel and copper, and the plated surfaces are plated with silver. Nickel plating, copper plating, or silver plating improves the joining strength between the first casing member and the second casing member. Silver plating can also reduce the losses due to the high frequency current flowing through the metal casing, due to its high electric conductivity. This arrangement, therefore, can provide a nonreciprocal circuit device with improved frequency characteristics.
According to another aspect of the present invention, there is provided a communication apparatus. Since this communication apparatus includes the nonreciprocal circuit according the present invention, it has the same advantages as those of the nonreciprocal circuit device according to the present invention.